The present invention relates to a shifting mechanism of the "X-Y" type for use with a shift bar housing assembly of a mechanical change-gear transmission, and more particularly, to such a mechanism of the type which is automatically or semi-automatically shifted, on a transmission which otherwise would be, or could be, manually shifted.
Shift bar housing assemblies for mechanical change-gear transmissions usually comprise a plurality of generally parallel, independently transversely movable shift bars, which are to be selected, and moved transversely, to effect engagement/disengagement of a particular gear ratio. Such shift-bar housing assemblies are well known to those skilled in the art, as may be seen by reference to U.S. Pat. Nos. 2,951,392; 4,567,785; and 4,873,881, all of which are assigned to the assignee of the present invention and incorporated herein by reference.
Typically, such shift bar housing assemblies have been manually controlled, and operated by a shift finger fixed to a directly mounted shift lever, or to the cross-shaft of a remotely controlled shifting mechanism. Automatic and semi-automatic mechanical transmissions, based upon generally standard mechanical transmissions, but provided with electric or pneumatic actuators, are now well known in the prior art, as may be seen by reference to U.S. Pat. Nos. 4,361,060 and 4,648,290, the latter of which is assigned to the assignee of the present invention, and both are incorporated herein by reference.
Also now well known in the prior art are automatically controlled "X-Y" type shifting mechanisms, wherein a shift finger is automatically moved in the axial (X--X) or rail selection direction, and then in the transverse (Y--Y) or gear engagement/disengagement direction. It will be understood by those skilled in the art that reference herein to the "transverse" and "axial" directions pertains to the shifter, and does not necessarily imply any particular direction on either the transmission or the vehicle. Also known are single shaft shifting mechanisms, as may be seen by reference to U.S. Pat. No. 4,920,815, which is assigned to the assignee of the present invention, and the disclosure of which is incorporated herein by reference.
Frequently, such an automatic or semi-automatic X-Y shifting mechanism is applied to a non-synchronous mechanical change-gear transmission. As used herein, the term "non-synchronous" will be understood to refer to a mechanical change-gear transmission of the type not equipped with synchronizers (or with less than extremely accurate synchronizers).
In a typical X-Y shifting mechanism, there is a strategy for gear preselection which involves applying a preload force (e.g., 20 to 40 lbs.) to the sliding clutch, but which lets the clutch remain "in gear" (the gear-engaged position) until a torque break or reversal occurs, at which point the clutch moves from the "in gear" position to "out of gear" (the gear-disengaged position).
Although shifting mechanisms of the type described above have been generally acceptable, and their operation generally satisfactory, there are certain disadvantages associated with such shifting mechanisms. A primary problem is that such shifting mechanisms require a substantial amount of control logic in order to achieve the proper timing of the various shifting operations, and the desired shift response. Such control logic adds substantially to the cost and complexity of the overall shifting mechanism.